net: do not call time.Now() twice

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/104080043

src/pkg/net/hosts.go

@@ -51,7 +51,7 @@ func readHosts() {
 			}
 		}
 		// Update the data cache.
-		hosts.expire = time.Now().Add(cacheMaxAge)
+		hosts.expire = now.Add(cacheMaxAge)
 		hosts.path = hp
 		hosts.byName = hs
 		hosts.byAddr = is